The invention relates to a device for scanning a surface comprising optically detectable marks along a scan line, which device comprises a radiation source for emitting a radiation beam, an objective system for guiding the radiation beam to the surface, a radiation-sensitive detection system for receiving radiation from the surface and an electronic circuit for processing output signals of the detection system.
The measurement of optical aberrations has recently become relevant in the field of optical recording, in particular the measurement of spherical aberration. The information density on optical record carriers may be increased by increasing the numerical aperture (NA) of the radiation beam used for reading and writing information on the record carrier. The record carriers are often scanned through a transparent layer protecting the information layer of the record carrier. A small variation of the thickness of the transparent layer causes a substantial change in the spherical aberration incurred by a high-numerical aperture radiation beam traversing the transparent layer. This spherical aberration may be reduced by using a dual lens objective system. Such a system has a first lens and a second lens, the second lens being a piano-convex lens arranged between the first and lens and the record carrier, and a small spacing between the piano surface and the record carrier. In some applications the plano-convex lens is referred to as a solid immersion lens.
The article xe2x80x9cHigh density optical disk system using a new two-element lens and a thin substrate diskxe2x80x9d by F. Maeda et al, published in the proceedings of ISOM96 p. 342-344 discloses an optical recording system having such a dual-lens objective system. The spherical aberration due to variations in the thickness of the transparent layer are compensated by changing the axial position of the piano-convex lens of the objective system. The system determines the spherical aberration in the beam reflected from the record carrier and uses this value to position the piano-convex lens. The spherical aberration is determined from the shape of the focus error signal as a function of the focus error. The axial position of the plano-convex lens is optimized to obtain the desired shape. The method has as a disadvantage that the shape of the focus error signal as a function of the focus position must be analysed, which requires wobbling the objective system through the point of best focus. During wobbling the reading and writing performance of the optical disk system is reduced.
It is an object of the invention to provide an aberration detection system that does not have the above disadvantages. It is another object of the invention to provide a device forming a more accurate focus error signal.
This object is met by a device as described in the preamble, which device is characterized according to the invention in that the detection system comprises a plurality of detectors, each detector having an output for providing a detector signal, and in that the device comprises an electronic circuit for forming a time difference between corresponding parts of the detector signals relating to passage of the radiation beam over one of the marks and for generating from the time difference a signal representing a wavefront aberration of the radiation beam.
The invention is based on the insight that different rays within the radiation beam will behave differently when the wavefront of the radiation beam deviates from the shape required for forming a proper focal spot on the surface. Such a deviation occurs when the beam is affected by optical aberrations. In particular, the position at which a ray is incident on the surface or information layer depends on the position of the ray in the pupil of the beam. A ray which impinges on the information layer ahead of the central part of the focal spot will experience the presence of a mark in the layer earlier than the rays forming the central part of the focal spot. It turns out that a suitable division of the detection system in detectors allows measurement of the time difference between the rays coming from the surface. The different detectors will determine different passage times for leading and trailing edges of the marks. A measurement of the time difference between the detector output signals of the occurrence of a particular feature of the surface, such as a leading or trailing edge of a mark, allows a determination of the primary optical aberrations.
The second object of the invention is met by a device as described in the preamble, which device is characterized according to the invention in that the detection system comprises eight detectors arranged in four quadrants, each quadrant being split at a radius in an inner part and an outer part, each detector having an output for providing a detector signal, and in that the device comprises an,electronic circuit for forming a time difference between corresponding parts of the detector signals relating to passage of the radiation beam over one of the marks and for generating from the time difference a focus error signal.
Since the device measures time differences in the scan direction, the wavefront of the beam must deviate from spherical in a plane containing both the axis of the beam and the scan line in order to measure non-zero time differences. Defocus, spherical aberration and tangential coma are examples of wavefront deviations that can be measured when scanning along the scan line. Other wavefront deviations, such as transverse coma, can be determined by the same method if the focal spot is wobbled in a direction transverse to the scan line and the detection system has a dividing line substantially parallel to the scan line. A measurement of the defocus in two directions allows the determination of the value of astigmatism.
The invention further relates to a method for determining the focus error and the optical aberrations from a time or phase measurement.
The invention also relates to a record carrier having specific patterns of marks located at specified parts of the tracks.
It is remarked that U.S. Pat. No. 4,051,527 describes a device that measures time differences between output signals of detectors and uses these differences to form a signal indicative of the distance between the centre of the focal spot and the track to be followed. In contrast to the present invention, the signal of the known device is relatively insensitive to wavefront deviations of the radiation beam.